The invention relates to a device for automatically determining the deviation between the structures of a pattern and those of a reference object.
The structures to be investigated with respect to their mutual deviation may be present, for example, on the masks which are customary in semiconductor production or also on the wafers produced by exposure processes. However, other microstructures can also be involved, which are applied to workpieces for the examination of their accuracy to gauge.
The requirements imposed by the semiconductor industry on the accuracy of the measuring devices for research, development and production control increase to the extent that the masks and wafers become larger and their structures smaller. The important matter is to supervise the production process and to correct that process in due time, in order to achieve an optimal output.
An economic process for the monitoring of the accuracy to gauge of masks and wafers from the current production is their comparison with a master mask. The mask comparison devices (e.g. LEITZ list item no. 810-109) developed for this purpose permit a visual observation of superposition errors between the structures and a visually monitored measurement of the superposition errors. By means of micrometer screws, the structures to be compared are displaced relative to one another until such time as their edges are disposed one above the other. This process is costly in terms of time, and is not free from subjective error effects caused by the person carrying out the measurement.
From German Offenlegungsschrift No. 3,305,014, an arrangement is known, by means of which superposition errors can be determined automatically by a photometric measurement. As in the case of the visually monitored mask comparison devices, the structures to be compared are illuminated in complementary colors and are imaged when superposed. The superposed image of the structures is transferred into the plane of a measurement gap and moved relative to the latter. The energy of the light passing through the measurement gap is broken down into light-energy components of each respective one of the complementary colors, corresponding to the differing wavelengths. A signal curve is generated in each instance from the light-energy components, as a function of the travel coordinates of the image relative to the measurement gap. The edge location of the structures to be compared with one another is determined from these signal curves in accordance with processes, known per se, of photometry for the determination of structure widths, and the superposition error of the two structures relative to one another is computed therefrom by difference formation. This automatically operating measurement arrangement requires the generation of two separate image channels, which are distinguishable by complementary color recognition.
While wavelength-dependent imaging differences in the two image channels can in part be disregarded in the visual observation of the mixed image, because of the limited spectral sensitivity and the resolving power of the eye, they are extremely important for the purposes of a photometric measurement, which is also intended to provide an increase in the accuracy of measurement. For the purposes of a separation, according to measurement technique, of the two image channels, the optical imaging systems must in the first instance be optimally corrected for the colors to be transmitted by them. However, this can be achieved only in the part of the imaging beam path in which the image channels extend separately. Accordingly, in the part of the imaging beam path in which the two image channels are transmitted while superposed it is necessary to find a compromise with regard to the chromatic correction. This is the more successful, the smaller is the wavelength difference of the two complementary colors. Thus, there is however, in turn, a difficulty on account of the color filters to be employed for the color separation. These have only a limited characteristic gradient, so that there is a certain spectral overlap region. For the purposes of the photometric measurement, this means cross-talk from one of the image channels into the other.
Additional difficulties arise as a result of the differing spectral sensitivity of the receivers, which leads to a non-uniform signal-to-noise ratio in the two measurement channels. This renders the subsequent signal processing more difficult.
A further serious disadvantage of the color recognition of the image channels is due to light-energy considerations, since the limitation of bandwidth necessarily restricts the utilized spectral range of the light source. The better is the color splitting generated by the cut-on filters, the greater are the light losses, which cannot be used for the measurement. In the case of rapid scanning of the structures, the light intensities remaining for the individual spectral regions are no longer sufficient to obtain a good signal-to-noise ratio. However, an increase in the total light intensity leads, in some cases, to local heating of the objects, which may lead to destruction of the structures.
The contrast ratios determined by the object are particularly critical for the purposes of a color characteristic. Differing coloration of the structures, coating layers, mask materials, wafer layers etc. lead to differing color contrasts within the individual image channels and also in comparison of the two image channels with one another.
Since, in the case of an automatic measurement, a facility for visual observation of the respective measurement region must also in all cases be possible, there are, in practice, additional restrictions with regard to the optimization of the individual effective parameters. Accordingly, the complementary colors red and green are usually selected for the color characteristic.